1) Field of the Invention
The present invention relates to a technology in which there is a gap between a latent image carrier and a charger, which charges the latent image carrier.
2) Description of the Related Art
In an image forming apparatus like a copying machine, a printer, a facsimile, a printing machine etc., an electrostatic latent image is formed on the latent image carrier according to information from a stepper, a host computer etc. The electrostatic latent image is then visualized and the visualized image is transferred to a recording medium like a paper. The image is then fixed and output as either as a copy or an original.
A surface of a photosensitive layer of the latent image carrier is uniformly charged with a charger, which is generally a roller (hereinafter, “charging roller”), before forming the electrostatic latent image. Two charging methods are known; non-contact charging and contact charging. In the non-contact charging, the charging roller does not make a physical contact with the latent image carrier. The non-contact charging is also called corona charging.
On the other hand, in the contact charging, a charging roller makes a physical contact with the latent image carrier. In the non-contact charging, a minute gap is set between the latent image carrier and the charging roller and a discharge is caused to occur in the minute gap. For example, Japanese Patent Application Laid Open Publication No. 1991-240076 discloses the non-contact charging.
The contact charging has an advantage over the non-contact charging in that less quantity of ozone gas is generated in the charging process. However, in the contact charging, since the charging roller touches the latent image carrier, residual toner, paper dust etc. easily gets stuck to the charging roller and causes uneven charging of the latent image carrier. Uneven charging of the latent image carrier leads to degraded image. In this respect the non-contact charging in preferable over the contact charging.
Two methods are widely used to apply a charging bias to the charging roller, whether it be the contact charging or the non-contact charging; AC-application method and DC-application method. In the DC-application method, a DC voltage, which is constant voltage controlled, is applied to the charging roller. In the AC-application method, an AC voltage, which is constant voltage controlled, is superimposed on a DC voltage, which is constant voltage controlled, and the resultant voltage is applied to the charging roller.
The charging bias applied to the charging roller varies depending on the physical properties, for example, the surface resistance, of the charging roller. The physical properties of the charging roller change with the environmental conditions, for example, humidity, temperature, around the charging roller. Japanese Patent Publication Nos. 3154628, 1997-120199 disclose changing the charging bias applied to the charging roller depending on the environmental conditions around the charging roller.
The non-contact charging has a typical problem that the width of the gap between the charging roller and the latent image carrier changes with the environmental conditions around the gap. The change in the width of the gap is due to expansion or reduction of the charging roller, the latent image carrier, or a spacer, used to maintain the gap, due to a change in the humidity or the temperature around the gap. The change in the gap is difficult to deal with because the charging roller, the latent image carrier, and the spacer expand or reduce differently at different locations. If the width of the gap at different locations is different, the charging becomes uneven and that degrades the quality of the image.